Train speed control system



18, 1955 J. E. FREEHAFER TRAIN SPEED CONTROL SYSTEM 3 Sheets-Sheet 2Filed Nov. 16, 1951 MON DQQE

EEEE 1 nnEJsE II INVENTOR. By J. E. FREEHAFER QESQUWQ His ATTORNEY 1955J. E. FREEHAFER TRAIN SPEED CONTROL SYSTEM 3 SheetsSheet 3 Filed Nov.16, 1951 r l I I l .l l NEE nnfiisr 45x5 m 5:527. l l I L INVENTOR.

I I I L must nnvm ks 4829C. m mv JEFPEEHAFEQ His ATTORNEY United StatesPatent M TRAIN SPEED CONTROL SYSTEM John E. Freehafer, Rochester, N. Y.,assignor to General Railway Signal Company, Rochester, N. Y.

Application November 16, 1951, Serial No. 256,628

19 Claims. (Cl. 246182) This invention relates to train speed controlsystems for railroads, and it more particularly pertains to frequencyresponsive speed governing means for a train, together with circuitmeans for checking the integrity of the system;

In automatic train control systems it has been common practice to employa centrifuge speed contactor mounted on a journal box of a locomotive asa part of a train speed control system. In such case, the contactor issubject to severe vibration, and circuit selections through therespective contacts have to be made by several different circuits havingwires extending from the other train control equipment at a remote pointon the locomotive to the contactor.

Generally speaking, and without attempting to define the scope of thepresent invention, the present invention contemplates the driving by anaxle of a locomotive of a frequency generator, such for example as thegenerator disclosed in the application of O. S. Field, Ser. No. 256,634,filed November 16, 1951, which has resulted in Pat. No. 2,651,734, datedSeptember 8, 1953. The generator output is fed to apparatus at a remotepoint on the locomotive where it is used through suitable amplifier andfilter stages to govern an electropneumatic valve (commonly known as anEP valve) so as to cause a service application of the brakes in case ofexcessive speeds in speed restricted territory.

Because of the circuits for the control of the EP valve involvingsafety, it is desirable that the integrity of these circuits be checkedand thus continuity of the'safety circuits is checked by a check circuitto which energy is applied by an oscillator at a selected frequency.This oscillator energy is inductively applied to the output winding ofthe generator on the axle of the locomotive, from which it feeds throughthe safety circuits to check their integrity. In one form of theinvention, the oscillator energy is of a frequency to pass through aselected high-pass filter, and by reason of there being continuity inthe circuit, the oscillator output is applied intermittently, and it isonly the intermittent energization of the check circuit that can beeffective to maintain the EP valve energized, thus preventing anautomatic application of the brakes. This intermittent operation isefiective when the train is standing still, as well as when it is inmotion.

If the speed of the train exceeds the speed permitted in a restrictedarea, the generated frequency is above the beginning of the pass band ofthe selected high-pass filter, and thus energy from the generator is fedthrough the filter to maintain steady energization of the detectorrelay, which is normally intermittently operated, and thereby set up adistinctive condition of energization which causes the deenergization ofthe EP valve. The check circuit, by its intermittent energization, iseffective when the locomotive is stopped to prevent actuation of the EPvalve.

An object of the present invention is to provide a purely electricalsystem for the selective control'of an 2,721,258 Patented Oct. 18, 1955EP valve in accordance with the output of a frequency generator coupledto a locomotive axle, and in accordance with permitted speeds of trainsthrough selected different restricted speed areas.

Another object of the present invention is to maintain a constant checkof the integrity of an electrical speed indicating system for theselective control of an EP valve.

Another object of the present invention is to employ the means forchecking the continuity of the speed indi-' cating system as a means formaintaining the EP valve energized when the train is not in motion, andthus when there is no voltage generated by the frequency generator whichis driven by the locomotive axle.

Other objects, purposes, and characteristic features of the presentinvention will be in part obvious from the accompanying drawings, and inpart pointed out as the description of the invention progresses.

In describing the invention in detail, reference is made to theaccompanying drawings in which corresponding parts are identified bysimilar reference characters; and in which;

Fig. l is a schematic diagram illustrating a system for the control ofan EP valve in accordance with the speed of a train wherein the EP valveis maintained normally energized by a check circuit, and is deenergizedby the generated frequency increasing to a point above the frequency ofa selected high-pass filter;

Fig. 2 is a schematic diagram illustrating a system for the control ofan EP valve in accordance with the speed of a train wherein the EP valveis maintained energized when the train is in motion by the output of afrequency generator, but its energization is prevented by a selectedlow-pass filter when the speed of the train exceeds a particularselected restricted speed limit;

Fig. 3 illustrates typical detail circuits which may be employed for theoscillator and amplifier respectively indicated in block form in theorganization according to Fig. 1;

Fig. 4 illustrated a form of high-pass filter that is satisfactory foruse in a system provided according to the present invention;

Fig. 5 illustrates the organization of a low-pass filter suitable foruse in a train control system such as that herein disclosed; and,

Fig. 6 is a sequence diagram illustrating the general mode of operationof the system according to the embodiment illustrated in Fig. 1.

The illustrations employed in the disclosure of the present inventionhave been arranged to facilitate the disclosure as to the mode ofoperation and the principles involved rather than for the purpose ofillustrating the construction and arrangement of parts that would beemployed in practice. Thus the various parts are illustrated byconventional symbols, some parts well known in the art being shown byblock diagram. The symbols and are employed to indicate connections tothe positive and negative terminals, respectively, of suitable batteriesor other sources of direct current, and the symbols (B-|-) and (B-) havebeen used to designate connections to a suitable high voltage directcurrent B supply for electronic tube circuits.

FIG. 1 APPARATUS suitable amplifier for the control of a code followingrelay CR. A slow acting front contact repeater relay CRFP a is providedfor energization in accordance with the pulsing of front contact 41 ofrelay CR, and a low speed control slow drop away relay 75R is normallyenergized whenever a code is being received so that the relay CR isactive. A medium speed decoding relay 120R is energized through anassociated filter and rectifier unit designated as 120DU only upon thereception of a 120 code rate, and similarly a relay 180R is provided tobe energized only in response to a 180 code rate through an associatedfilter and rectifier unit 180DU. This code receiving equipment carriedby the locomotive will be readily recognized as conforming to generalpractice, and details which have not been shown with respect to theorganization of this apparatus can be readily supplied by those skilledin the art, particularly in view of the teachings of several priorpatents, such, for example, as the U. S. patent of W. H. Reichard, No.2,223,131, dated November 26, 1940.

The above described apparatus provides information indicative of thespeed restrictions that should be imposed because of trafiic conditionsas set up by track circuit codes transmitted through the track rails,and this information is used in train control systems of the typedisclosed in the above mentioned Reichard patent for comparison withmeans indicating the actual speed of a locomotive to determine as towhether the speed of the locomotive is within the limits indicated bythe rate of code received.

According to the present invention, the axle 11 of the locomotive drivesa suitable frequency generator, such, for example, as one of thefrequency generators disclosed in the patent application of O. S. Field,Ser. No. 256,634, filed November 16, 1951. The frequency generatoremployed, however, may be of any suitable structure to generate afrequency directly variable with the speed of rotation of the axle 11.Thus a suitable generator can be constructed according to thediagrammatic showing of the generator G in Fig. 1, employing a U-shapedmagnetic structure having soft iron pole pieces 14 and 15, a permanentmagnet yoke PM across the ends of the pole pieces 14 and 15, and aslotted magnetic shunt 16 across the pole pieces 14 and 15 between thepermanent magnet PM and a toothed rotor 17. One requisite of a generatorof this nature is that it should be constructed to produce an output ata relatively low speed of the axle 11, and the efiiciency of such agenerator at a low speed is particularly improved by the use of amagnetic shunt structure such as that diagrammatically illustrated inFig. 1, or such as that specifically described in the above mentionedapplication of O. S. Field. The generator G according to Fig. 1 has twowindings 18 and 19 about core structure which also serves as themagnetic shunt 16, the winding 18 being used as a primary winding of atransformer to induce a generated checking voltage in the second winding19 for checking circuit continuity and for maintaining the systemproperly active when the locomotive is not in motion, this secondarywinding 19 being the winding from which an output of the generator G istaken.

The control of the brakes of the locomotive is ac complished by theselective energization or deenergization of an electro-pneumatic valveEPV, the structure 01 which is well known to those familiar with the artand is more specifically disclosed, for example, in the patent to C. S.Bushnell, No. 1,855,596, dated April 26, 1932. The electro-pneumaticvalve EPV is normally energized to permit the brakes of the train to bereleased, and is effective when deenergized to reduce the air pressureand thereby cause application of the brakes of the train.

A slow acting penalty relay PR is provided for controlling directly theenergization of the valve EPV, and its control is in turn dependent upona slow drop away relay C and an air reduction switch SAS. It is to beunderstood, and it will be more readily apparent as the descriptionprogresses, that the present invention can be readily applied inassociation with practically any penalty and/or acknowledgmentorganization, and should not be considered limited by the penalty meansherein disclosed.

The energization of a slow drop away control relay C for governingenergization of the penalty relay PR is dependent upon the intermittentdischarge of a condenser 36 through the windings of the relay C, theintermittent or cyclical charge and discharge of the condenser 36 beinggoverned by a relay DP which is illustrated as having both slow pickupand slow drop away characteristics.

The intermittent operation of the relay DP is in turn dependent upon theintermittent operation of a detector relay D, which is energized as aresult of the output of the generator G, as applied through a suitableamplifier and through a selected high pass filter L.MPH, MMPH, or HMPH,and through a rectifier 51. Although it is to be understood that thestructure of the amplifier and of the high pass filters LMPH, M-MPH, orH-MPH may be a matter of choice in accordance with the requirements ofpractice, typical amplifier and filter circuit organization have beenillustrated in Figs. 3, 4, and 5. Thus according to Fig. 3, aconventional push-pull amplifier is illustrated as having an input fromthe winding 19 of the frequency generator G and having an output appliedfor the control of relay D through a selected high pass filter LMPH,MMPH, or H-MPH. Figs. 4 and 5 illustrate specifically suitable typicalhigh and low pass filters respectively, employing m-derived endsections.

A suitable oscillator is provided for generating a check frequency foruse in checking the integrity of the system for the control of the relayD, and also for providing for energization of relay D when thelocomotive is not in motion. Although various types of oscillators maybe employed, with reference to Fig. 3, a typical organization employinga resistance stabilizer oscillator is illustrated for example.

FIG. 2 APPARATUS For convenience in simplifying the description of theembodiment of the invention illustrated in Fig. 2, those parts of Fig. 2which correspond with the structure illustrated and described withrespect to Fig. 1 are identified by corresponding reference characters,and reference is to be made to the description relative to Fig. 1 fordetail consideration of these parts. The generator G1 illustrated inFig. 2 can also be of the type disclosed in the above mentionedapplication of O. S. Field, or as illustrated in Fig. 1, but it is showndiagrammatically as having pole pieces and 31 connected by a permanentmagnet PM1 as a back yoke, the pole pieces cooperating with respectiveteeth of a toothed rotor 32 which is driven by the axle 11 of thelocomotive. The generator G1, like the generator G of Fig. 1, hasrespective primary and secondary windings, the primary winding 33 beingsubject to energization by a suitable oscillator generating a frequencyf1, and the secondary winding 34 being subject to an induced voltagefrom the primary winding 33, and being used for providing the outputfrom the generator upon rotation of the toothed rotor 32.

Although for simplification of the disclosure of the form of theinvention illustrated in Fig. 2, no penalty means is provided, it is tobe understood that a penalty relay PR can be employed in accordance withthe requirements of practice for general purposes corresponding to thoseto be hereinafter more specifically considered with reference to the useof the penalty relay PR in the form of the invention illustrated inFig. 1. Fig. 2, a detector relay D1 which is governed by the output ofan amplifier through a selected low-pass filter provides directly forthe energization of the electro-pneumatic valve EPV when the train is inmotion, and a slow drop away relay C1 is effective to provide for theenergization of the valve EPV when the train is not in motion. The relayC1 is a capacitor energized relay comparable to the relay C of Fig. 1,and the intermittent charge and .dis-

Thus according to charge of the condenser 35 associated with the relayC1 is governed by a relay DSP having slow pickup and drop awaycharacteristics, which in turn is governed by a relay D8 which issubject to energization, through a suitable pre-amplifier and rectifier,from the winding 34 of the generator G1.

In the embodiment of the invention according to Fig. 2, a speedindicator SP is illustrated as being governed by the output of thegenerator G1 through a suitable filter for removing the oscillatorfrequency 1, and through a frequency-current converter, which is adaptedto convert frequency to current in a direct ratio so that thespeedindicator SP is driven in accordance with the speed of the axle 11.The apparatus comprising the frequencycurrent converter is well known tothose familiar with the art and it can be provided, for example, asdisclosed on pages 264 and 268 of the January 1945 issue of Electronicsmagazine. Although the speed indicator has been illustrated only in theembodiment illustrated in Fig. 2, it is to be understood that it can beused in connection with the apparatus provided according to Fig. 1, ifsuit-' able means is employed to filter out the frequency of theoscillator, and it is to be further understood that the use of the speedindicator is optional in any form of the present invention.

Having thus considered the general organization of theapparatusaccording to the present invention, a more specific consideration willhereinafter be given of the circuit organizations when considering themode of operation of the system under certain typical operatingconditions.

Operation To facilitate an understanding of the specific mode ofoperation as it will be hereinafter considered, consideration will firstbe given to the general mode of operation of the organization accordingto Fig. 1, without specific reference to the circuit organizationsinvolved.

The general mode of operation of the track code received apparatus iswell known to those familiar with the art wherein the track code isreceived by the receivers 12 and 13 disposed over the respective trackrails on the front of the locomotive, the code pulses induced in thesereceivers being suitably amplified and filtered and effective to causethe pulsing of the code following relay CR. Relays CRFP and 75R aremaintained steadily picked up in accordance with the pulsing of therelay CR, and if the code received is at a 120 or a 180 rate, thepulsing of contact 43 of relay CR is effective to energize the relay120R or relay 180R in accordance with the code that is being received.Thus the relay 75R when picked up is indicative of the reception of atrack circuit code of any rate, relay 120R when energized is indicativeof the reception of the 120 code, and relay 180R is energized inaccordance with the reception of a 180 track circuit code.

These relays when selectively energized will be readily recognized asbeing indicative on the locomotive of the speed restrictions intended tobe applied to the train for respective areas along the trackway. If onlythe relay 75R is picked up, this is indicative of a low speed area, andthus the train must be operated within a low speed limit. Similarly therelay 120R when picked up with the relay 180R dropped away defines amedium speed limit, and the relay 180R when picked up with the relay120R dropped away establishes a high speed limit.

When the locomotive is not in motion, or when the locomotive speed isbelow the limit defined by the track circuit code receiving apparatus,the detector relay D (see Fig. 1) is pulsed in accordance with the slowacting char acteristics of its repeater relay DP which selectively opensand closes the output circuit of the oscillator. The relay DP must bepulsed in order to maintain the relay C picked up, which in turnenergizes the penalty relay PR; and through the front contact 66 of thepenalty relay PR, the valve EPV is maintained energized, and thuspermits the brake pressure of the train to be maintained. The relay D isenergized by the oscillator, because the oscillator frequency is alwaysselected as being just above the frequency of the selected high-passfilter. In other words, the relay D is always subject to energization bythe output of the oscillator as applied through the windings of thegenerator G, acting as a transformer, and through a suitable amplifierand a selected high-pass filter, irrespective of the speed at which thelocomotive is being operated.

Also through a part of this same circuit organization, it will bereadily apparent that the output of the generator G is applied to thecircuit organization for the control of the relay D, and is preventedfrom energizing the relay D only when the frequency generated is lowerthan the frequency of the particular high-pass filter that has beenselected by the train control receiving apparatus. It will thus be seenthat as long as the frequency generated is below the frequency of theselected high-pass filter, no appreciable amount of energy can beapplied,

from the generator to the control of the relay D, and thus the relay Dis permitted to be pulsed in accordance with the operatingcharacteristics of its slow acting repeater relay DP.

Assuming the above described mode of operation wherein it has been setforth that the pulsing of the relay D is required in order to maintainthe valve EPV energized; it will be apparent that the output of thegenerator G is prevented from energizing the relay D solely by reason ofthe generated frequency being below the pass band of the selectedhigh-pass filter, thus it will be seen that when the frequency generatedby the generator G becomes high enough to fall within the pass band ofthe high pass filter that has been selected, the relay D becomesenergized by the output of the generator, and therefore is maintainedsteadily picked up, resulting in the deenergization after a time of thevalve EPV. Because of the relay D failing to be pulsed, its repeaterrelay DP is maintained steadily energized, and thus the control relay Cis maintained steadily deenergized, and the dropping away of this relayeffects the dropping away of the penalty relay PR (assuming that therehas been no air reduction manually applied), and the relay PR whendropped away causes the actuation of the valve EPV for a serviceapplication of the brakes of the train.

Having thus described the general mode of operation of the embodiment ofthe invention illustrated in Fig. l, a

more detailed consideration will now be given as to the.

circuit organizations involved in providing the above described generalmode of operation.

It has been pointed out that the oscillator employed for generating acheck frequency can be of any suitable type such, for example, as of theresistance stabilizer type specifically illustrated in Fig. 3. Thisoscillator has its frequency determined by a tuned circuit including aninductance 45 selectively coupled in multiple with a condenser 20, 4G,or 60 as selected by the code receiving.

relays 75R, R, and R. The condenser 20 is used to govern the frequencywhen it is connected in multiple with the inductance 45 through frontcontact 46 of relay 75R, back contact 47 of relay 120R and back contact48 of relay 180R.

Similarly, the condenser 40 is connected in multiple with the inductance45 to govern the frequency of the oscillator when relay 120R isenergized so that condenser 40 is connected to the inductance 45 throughfront contact 46 of relay 75R, front contact 47 of relay 120R, and backcontact 49 of relay 180R.

The frequency generated by the oscillator is determined,

by the connection of the condenser 60 in multiple with the inductance 45when the relay 180R is picked up so that the condenser 60 is connectedin multiple with the inductance 45 through front contact 46 of relay75R, back contact 47 of relay 120R, and front contact 48 of relay 180R.As has been heretofore pointed out, the frequency of the oscillator isadjusted by these selected circuits to be just within the pass band ofthe respective high pass filters L-MPH, M-MPH, and HMPH, respectively.The output of the oscillator is applied to the winding 18 of thefrequency generator G through back contact of relay DP.

With reference to Fig. 3, the output winding 19 of the generator G isconnected directly to the input of a conventional push-pull amplifier.This amplifier can be of any suitable type so as to provide output powersufiicient to insure proper operation of the detector relay D at timeswhen the output frequency is high enough to fall within the pass bank ofthe selected high-pass filter in the circuit for the control of thedetector relay D.

With reference to Fig. 1, one of the output wires is connected toground, and the other is connected through contact selections andthrough the rectifier 51 to the winding of relay D. Thus, if it isassumed that the relays 75R and 180R are picked up, in accordance withthe reception of the 180 track circuit code, the circuit for the controlof relay D in accordance with the output of the amplifier is throughfront contact 52 of relay PR, front contact 53 of relay 75R, backcontact 54 of relay 129R, front contact 55 of relay 180R, high-passfilter H-MPH, front contact 56 of relay 189R, back contact 57 of relay120R, front contact 58 of relay R, front contact 59 of relay PR, andrectifier 51.

If the relay R is picked up instead of relay R, in accordance with thereception of the 120 track circuit code, the circuit just described forthe relay D is open at front contacts 55 and 56 of relay 180R, and therelay D can be energized through front contact 52 of relay PR, frontcontact 53 of relay 75R, front contact 54 of relay 120D, back contact 61of relay 180R, filter MMPH, back contact 62 of relay 180R, front contact57 of relay 120R, front contact 58 of relay 75R, front contact 59 ofrelay PR, and rectifier 51.

If it is a 75 track circuit code that is being received, the circuitjust described for the relay D is open at front contacts 54 and 57 ofrelay 120R, and at front contacts 55 and 56 of relay 180R, but the relayD can be energized through front contact 52 of relay PR, front contact53 of relay 75R, back contact 54 of relay 120R, back contact 55 of relay180R, filter LMPH, back contact 56 of relay 180R, back contact 57 ofrelay 120R, front contact 58 of relay 75R, front contact 59 of relay PR,and rectifier 51.

Relay D when picked up closes an obvious circuit at front contact 63 forthe energization of its slow acting repeater relay DP, and the pickingup of the relay DP opens the output circuit of the oscillator at backcontact 50. solely dependent for energization upon the oscillator, aswhen the speed of the train is below the limit determined by the trackcircuit code receiving apparatus in combination with the high-passfilters, the opening of contact 5t) deenergizes the relay D and thuscauses relay D to be dropped away, and thus causes the subsequentdeenergization of relay DP by the opening of front contact 63.

It is thus provided that the picking up of relay DP causes the droppingaway of relay D, which in turn causes the dropping away of relay DPafter a time determined by the slow acting characteristics of relay DP.In this manner, it will be seen that when the speed of the train isbelow the predetermined allowable speed limit, the relay D isintermittently operated, together with its repeater relay DP, and by thepulsing of contact 64 of relay DP, the c0ntrol relay C is maintainedpicked up by the intermittent discharge of the condenser 36 (condenser36 being charged through front contact 64 of relay DP). By the closureof front contact 65 of relay C, relay PR is maintained picked up, andthe relay PR is effective when picked up to apply energy through itsfront contact 66 to the valve EPV so as to permit the air pressure inthe brake control system to be maintained.

It will be noted from the organization that has been described, that themaintaining of the penalty relay PR It will thus be seen that if thedetector relay D is V picked up is dependent upon the relay C beingmaintained energized by the pulsing of contact 64 of the relay DP, andthus for normal operation of the train (without the brakes beingapplied) it is required that the relay DP be pulsed by its associateddetector relay D as a check that continuity is maintained for a circuitextending through the winding 19 of the generator G, through theamplifier, contact selections, and selected filter to the winding ofrelay D. Thus it is determined that the relay D is conditioned to besteadily energized at any time when the frequency generated by thefrequency generator G falls within the pass band of the particularhigh-pass filter selected to be included in the circuit in accordancewith the restrictions required by the track circuit code receivingapparat-us.

It it is assumed, for example, that while the detector relay D ispulsing in accordance with normal progress of the train below its speedlimit for the particular area it is passing through, consideration willnow be given to the condition where the speed of the train exceeds thespeed permitted so that the frequency of the output of the generator Gis above the frequency of the high-pass filters LMPH, M-MPH, or H-MPH(whichever one has been selected). When this happens, with the high-passfilter including m-end sections as is illustrated in Fig. 4, and as hasbeen heretofore described, immediately upon the frequency of thegenerator G exceeding the frequency of the selected high-pass filter,sufficient energy passes to the relay D from the generator G, throughthe filter, to maintain the relay D steadily energized.

As a margin of safety to insure that there will be sufficient energy tomaintain the relay D energized by the output of the generator G, it ispreferable that the relative voltages of the oscillator and generator Grespectively as applied to the input terminals of the amplifier besubstantially in the ratio of one to two respectively. In other words, amargin of safety is assured by the output voltage of the generator beingsubstantially twice the output voltage of the oscillator.

When the relay D becomes steadily energized, its repeater relay DPbecomes steadily energized because of the closure of front contact 63 ofrelay D, and thus the pulsing of contact 64 of relay DP ceases, with theresult that the relay C is dropped away, and the dropping away of thisrelay opens the circuit for the penalty relay PR at front contact 65.

With the relay C dropped away, a warning device (not shown) is actuatedin accordance with the usual train control practice to advise theengineer that his speed must be reduced. Subsequent to the actuation ofthe warning device, it may be required that the engineer actuateacknowledging means, or take other steps in accordance with therequirements of practice in order to forestall an immediate emergencyapplication of the brakes of the train. Such acknowledgment andsuppressed braking means can be provided in accordance with theteachings of any one of a number of different prior patents.

For the purpose of simplification of the disclosure of the presentinvention, only a very simple form of suppressed braking has beenillustrated as a typical example, and this is in the form of the serviceair reduction switch SAS which is actuated to close its contacts upon amanual application of the brakes so that the penalty relay PR can beheld energized by a stick circuit including contact 76 of the airreduction switch SAS, and front contact 71 of the penalty relay PR. Itis thus provided that the application of the brakes manually forestallsan automatic application by maintaining the penalty relay PR energizedso as to maintain energy applied to the valve EPV through front contact66.

If, on the other hand, there was no manual application of the brakeswhen the speed exceeded the speed limit, the contact 70 of the airreduction switch SAS would rave been open and thus the relay PR wouldhave been permitted to drop away, according to its slow drop awaycharacteristics, after the opening of its circuit at front contact 65 ofthe control relay C. Thus relay PR when dropped away would have openedthe circuit for the energization of the valve EPV at front contact 66 tocause the actuation of the valve to apply the brakes of the train in amanner Well known to those familiar with the art.

If it is assumed that the penalty relay PR has been dropped away tocause an emergency application of the brakes of the train, the high passfilters are shunted out of the circuit for the control of the relay D bya connection through back contacts 52 and 59 of relay PR, and thus therelay D is maintained picked up by energy from the generator G,irrespective of the frequency of the generated voltage, until the trainsubstantially comes to a stop so that there is insufficient voltagegenerated by the generator 'G to maintain the relay D energized. Thiscondition will be recognized as that being comparable to the mode ofoperation generally required in train control systems where an automaticapplication of the brakes has been applied, which requires the train tobe brought substantially to a stop before release of the brakes canbecome effective.

Thus, when the train reaches a very low speed so that the relay Dbecomes dropped away, the dropping away of its repeater relay DP by theopening of front contact 63 closes a circuit to connect the output ofthe oscillator to the Winding 18 of the generator G, and thus the relayD becomes picked up as a result of energy feeding from the oscillator,and the intermittent operation of the relays D and DP is resumed as hasbeen heretofore described, the relay D being energized at this time,however, through the shunt applied through back contacts 52 and 59 ofthe penalty relay PR, shunting the high-pass filters out of the circuit.

Upon the pulsing of contact 64 of relay DP, relay C becomes picked up,and the picking up of this relay is effective by the closure of itsfront contact 65 to cause the picking up of the penalty relay PR, whichin turn applies energy to the valve EPV through front contact 66 so asto permit the release of the brakes of the train. The penalty relay PRin picking up removes the shunt applied in the circuit for the relay Dacross the high-pass filters by the opening of back contacts 52 and 59,and the closure of front contact 52 and 59 re-establishes the circuitfor the normal operation of the relay D as'it has been heretoforedescribed.

It has been pointed out that the frequency of the oscillator is selectedby contacts of the track circuit code receiving apparatus on thelocomotive to correspond with, or fall just within the pass band of therespective selected high-pass filters. In this way, the integrity of thehighpass filters is checked by the oscillator on the side of safety inthat should the frequency of the selected highpass filter shift to ahigher frequency than that for which the filter was originally designed,the filter would not pass the frequency of the oscillator because theoscillator frequency would then be below the frequency of the highpassfilter. Under these conditions, the relay C would become dropped away,and would cause an automatic application of the brakes should thepenalty be applied.

It is particularly for this reason that the frequency of the oscillatoris selected by the track code receiving apparatus, rather than being afixed frequency sufiiciently high to fall within the pass band of thehighest speed high pass filter, H-MPH. It is to be understood, however,that if this feature of checking the integrity of the high-pass filtersis not considered to be necessary, the oscillator can be operated at anypredetermined frequency sufficiently high to fall within the pass bandof the highpass filter H-MPH.

The embodiment of the present invention illustrated in Fig. 2 disclosesa system by which similar principles of train control can be appliedusing low-pass filters rather 10 than high-pass filters, and employingthe principle of hav= ing the respective low-pass filters cut out theenergization of a detector relay D1 in case the frequency of thegenerator G1 becomes higher than the frequency passed by the selectedlow-pass filter. This arrangement is on the fail safe principle in thatsteady energization of the detector relay D by reason of a generatedvoltage of the generator G1 is always required for the energization ofthe valve EPV whenever the locomotive is in motion. However, thisorganization in itself does not provide for energization of the valveEPV when the locomotive is not in motion, and therefore the oscillatorillustrated in Fig. 2 as generating a frequency fl is used for pulsingthe relays DS and DSP by a mode of operation comparable to that whichhas been described for the pulsing of relays D and DP of Fig. 1, exceptthat the intermittent operation of these relays is effective principallyonly when the locomotive is not in motion.

More specifically, the closure of back contact 75 of the relay DSPconnects the output of the oscillator of Fig. 2 to the winding 33 of thegenerator G1, and a voltage is induced in the secondary winding 34 ofthe generator G1 which is applied through a pre-amplifier and throughrectifier 76 to the detector relay DS. Relay DS in picking up causes thepicking up of the relay DSP by the closure of front contact 77, relayDSP having slow pickup and drop away characteristics so as to provide bythe selective opening and closing of back contact 75 in the outputcircuit of the oscillator for the intermittent operation of the relaysDS and DSP. The pulsing of contact 78 of relay DSP alternately chargesthe condenser 35 and discharges it through the winding of relay C1 tocause the relay C1 to be picked up and close its front contact 79 toapply energy to the winding of the valve EPV.

It will be noted that the control relay C1 can be picked up to applyenergy to the valve EPV through front contact 79 only so long as therelay DSP is intermittently actuated, and thus when the locomotive is inmotion, the relay DSP fails to pulse, and the relay C1 is dropped awayto open front contact 79. The relay DSP fails to pulse because the relayDS is steadily energized by the output voltage of the generator G1 thatis generated due to motion of the train, and thus the front contact 77of relay DS applies energy steadily to the repeater relay DSP.

When relay DS is picked up, the closure of its front contact 80 appliesenergy from the output terminals of the pre-amplifier to the amplifierillustrated in Fig. 2, and the output of this amplifier is appliedthrough a selected low pass filter to the detector relay D1.

Thus, if a 75 code is being received, the output of the amplifier ofFig. 2 is applied to relay D1 through front contact 81 of relay 75R,back contact 82 of relay 120R,

back contact 83 of relay 180R, low pass filter L-MPH;

back contact 84 of relay 180R, back contact 85 of relay R, front contact86 of relay 75R, and rectifier 87.

If it is a 120 rate code that is being received, the output of theamplifier of the organization according to Fig. 2 is applied to therelay D1 through front contact 81 of relay 75R, front contact 82 ofrelay 120R, back contact 88 of relay R, low pass filter M-MPH, backcontact 89 of relay 180R, front contact 85 of relay 120R, front contact86 of relay 75R, and rectifier 87.

If it is a 180 code rate that is being received, the relay D1 of Fig. 2receives energy from the output of the amplifier through front contact81 of relay 75R, back contact 82 of relay 120R, front contact 83 ofrelay 180R, low pass filter H-MPH, front contact 84 of relay 180R, backcontact 85 of relay 120R, front contact 86 of relay 75R, and rectifier87.-

Relay D1 when picked up applies energy to the winding of the valve EPVthrough front contact 90 which is connected in multiple with frontcontact 79 of the relay C1.

It will be seen that because of the slow drop away charac- 1 1 teristicsof the relay C1, the detector relay D1 will have sufiicient time tobecome picked up prior to the dropping away of the relay C1 when thelocomotive is first put in motion, so that there is no interruption inthe energization of the valve EPV.

It will be apparent from the circuit organization that has beendescribed that the relay D1 is subject to deenergization whenever thespeed of the locomotive is such that the frequency of the generator G1exceeds the frequency of the low-pass filter selected in accordance withimposed speed restrictions of the track circuit code. Thus the droppingaway of the relay D1 under these conditions is effective to cause thedeenergization of the valve EPV to cause an emergency application of thebrakes.

Although no suppressed braking means has been illustrated in the systemaccording to Fig. 2, it is to be understood that any suitable system ofsuppressed braking and/ or acknowledgment familiar to those skilled inthe art may be employed, and that the organization involving the use ofa penalty relay PR of Fig. 1 could as well be applied to theorganization according to Fig. 2, except that the penalty relay wouldinterrupt the circuit to the detector relay D rather than shunt thefilters as in Fig. 1.

If the speed indicator SP is to be used as is illustrated in Fig. 2, thefrequency ii of the oscillator must be above the frequency generated bythe generator G1 at the highest speed of the train. Under theseconditions relay D1 is at no time subject to operation by energy fromthe oscillator, because this frequency fl is above the frequency of anyof the low-pass filters LMPH, MMPH, or H-MPH.

Having described a train speed control system with reference to twospecific embodiments, it is desired to be understood that the formsdisclosed have been selected to facilitate the disclosure of theinvention, particularly as to the principles involved, rather than tolimit the number of forms which the invention may assume, and it is tobe further understood that various adaptations, alterations, andmodifications may be applied to the specific forms shown to meet therequirements of practice, without in any manner departing from thespirit or scope of the present invention, except as limited by theappending claims.

What I claim is:

1. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a condenser, a slow acting relay effective when picked up tocause energization of said electro-pneumatic valve, said slow-actingrelay being subject to energization only by the discharge of saidcondenser, a high-pass filter, an oscillator effective to generate agiven frequency within the pass band of said high-pass filter, atwo-position control relay having contacts effective in one position tocharge said condenser and in the other position to discharge saidcondenser through the winding of said slowacting relay, andelectro-responsive means for intermittently energizing said two-positioncontrol relay from said oscillator through said high-pass filter andthereby maintain said electro-pneumatic valve energized, saidelectro-responsive means being effective to steadily energize saidtwo-position control relay when the frequency of said generator is abovethe frequency of said highpass filter and thereby cause the actuation ofsaid electrop'neumatic valve to apply the brakes of the train.

2. A train speed control system having train carried equipmentcomprising in combination, an axle driven frequency generator, anelectro-pneumatic brake control valve effective when deenergized tocause the application of the brakes of the train, a control relay forsaid brake control valve effective when picked up to maintain said brakecontrol valve energized, and effective when dropped away to cause thedeenergization of said brake control valve, electro-responsive receivingapparatus distinctively actuated in accordance with the code rate ofenergization of the track rails, a high-pass filter,

and electro-responsive means including said high-pass filter formaintaining said control relay steadily picked up to thereby energizesaid electro-pneumatic valve except when the frequency of the output ofsaid axle driven generator falls within the band pass of said high-passfilter.

3. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a plurality of filters, each filter being effective to blocka different range of frequencies, track circuit code receiving meansefiective to select one of said filters dependent upon the particulartrack circuit code rate receiver, a detector relay having a slow actingrepeater relay, an oscillator having its output applied to a winding ofsaid generator through a contact of said repeater relay, circuit meansincluding a winding of said generator and said selected one of saidfilters for energizing said detector relay and thereby actuate said slowacting relay to open said contact in the output of said oscillator, acontrol relay, circuit means for maintaining said control relay pickedup only provided that said slow acting relay is intermittently actuated,and circuit means dependent upon said control relay being picked up forenergizing said electro-pneumatic valve.

4. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a filter effective to block frequencies outside of aparticular predetermined frequency range, a detector relay having a slowacting repeater relay, an oscillator having its output applied to awinding of said generator through a circuit including a contact of saidslow acting relay, circuit means including a winding of said generatorand said filter for energizing said detector relay and thereby actuatesaid slow acting relay to open said contact in the output circuit ofsaid oscillator, a control relay, circuit means for maintaining saidcontrol relay picked up only when said repeater relay is intermittentlyactuated, and circuit means dependent upon said control relay beingpicked up for energizing said electro-pneumatic valve.

5. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a plurality of filters, each filter being effective to blocka different range of frequencies, track circuit code receiving meanseffective to select one of said filters dependent upon the particulartrack circuit code received, a detector relay having a slow actingrepeater relay, circuit means including said selected one of saidfilters for energizing said detector relay from the out put of saidgenerator, an oscillator having its output applied to said circuit meansthrough a contact of said repeater relay to thereby cause actuation ofsaid repeater relay intermittently except when said detector relay isenergized by the output of said frequency generator, a control relay,circuit means for maintaining said control relay picked up provided thatsaid repeater relay is intermittently actuated, and circuit means forenergizing said electro-pneumatic valve provided that said control relayis picked up.

6. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a plurality of high-pass filters respectively effective topass current above different frequencies, track circuit code receivingmeans effective to select one of said lters dependent upon theparticular track circuit code received, a filter having a lowerfrequency pass band being selected for a lower rate code, a detectorrelay having a slow acting repeater relay, circuit means including thefilter selected by said receiving means for energizing said detectorrelay from the output of said generator, an oscillator generating afrequency falling within the pass band of the selected high-pass filterhaving its output applied through a back contact of said repeater relayto said circuit means to check the integrity of the circuit means forthe energization of said detector relay from said generator, wherebysaid repeater relay is intermittently actuated by reason of energyoriginating at said oscillator, a control relay, circuit means formaintaining said control relay picked up provided that said repeaterrelay is intermittently actuated, and circuit means for energizing saidelectro-pneumatic valve provided that said control relay is picked up.

7. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a plurality of filters, each filter being effective to blocka different range of frequencies, train control receiving apparatuseffective to be conditioned by cooperative electrical control withwayside apparatus, said receiving apparatus being effective to selectone of said filters, a detector relay having a slow acting repeaterrelay, circuit means including the filter selected by said receivingapparatus for energizing said detector relay from the output of saidgenerator when the output frequency is high enough to fall within thepass band of the selected filter, an oscillator generating a frequencyabove the frequency of the selected filter having its output appliedthrough a back contact of said repeater relay to said circuit means tocheck the integrity of the circuit means for the energization of saiddetector relay from said generator, whereby said repeater relay isintermittently actuated by reason of energy originating at saidoscillator, a control relay, circuit means for maintaining said controlrelay picked up provided that said repeater relay is intermittentlyactuated, and circuit means for energizing said electro-pneumatic valveprovided that said control relay is picked up.

8. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a high-pass filter effective to pass frequencies above aparticular frequency comparable to a speed limit frequency of said axledriven generator, a detector relay having a slow acting repeater relay,circuit means including said filter for energizing said detector relayfrom the output of said generator when the frequency of the generator isabove the frequency of said filter, an oscillator generating a frequencywithin the pass band of said filter having its output applied through aback contact of said repeater relay to said circuit means to check theintegrity of the circuit means for the energization of said detectorrelay from said generator, whereby said repeater relay is intermittentlyactuated only when the frequency of said generator is below the passband of said filter, a control relay, circuit means for maintaining saidcontrol relay picked up provided that said repeater relay isintermittently actuated, and circuit means dependent upon said controlrelay being picked up for energizing said electropneumatic valve.

9. A train control system having train carried apparatus, comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train,

a plurality of high-pass filters for different frequencies comparable tofrequencies of said generator at predetermined train speeds, traincontrol receiving apparatus effective to be conditioned by cooperativeelectrical control with wayside apparatus, said receiving apparatusbeing effective to select one of said high-pass filters, an

oscillator generating a frequency within the pass band of I the selectedhigh-pass filter, a detector relay having a slow acting repeater relay,circuit means for energizing said detector relay from the output of saidoscillator through said selected filter provided that said repeaterrelay is actuated to a particular position, whereby said detector relayis intermittently energized at a rate determined by the slow actingcharacteristics of said repeater relay, said circuit means alsorendering said detector relay subject to energization from saidgenerator through said selected filter when the frequency of thegenerator is within the pass band of the filter, and electro-responsivemeans for maintaining said electro-pneumatic valve energized providedthat said detector relay is intermittently actuated.

10. In a train control system, train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumaticvalve effective when deenergized to cause application of the brakes ofthe train, a plurality of high-pass filters for different pass bandsstarting at the respective frequencies of said generator atpredetermined train speeds, an oscillator, train control receivingapparatus effective to be conditioned in response to wayside apparatus,said receiving apparatus being effective to select one of said high-passfilters, and said receiving means being effective to select a frequencyfor said oscillator substantially corresponding to the boundaryfrequency between pass and stop of the filter selected, a detector relayhaving a slow acting repeater relay, circuit means including the filterselected by said receiving apparatus for energizing said detector relayby the output of said generator, whereby said detector relay is pickedup by energy from said generator only when the frequency of thegenerator is within the pass band of the selected filter,- said circuitmeans also being effective to energize said detector relay by the outputof said oscillator through a contact of said repeater relay, and throughsaid selected filter, and electro-responsive means for maintaining saidelectro-pneumatic valve energized provided that said detector relay isintermittently actuated.

11. In a train control system, train carried apparatus comprising incombination, an axle driven frequency generator, an electro-pneumatic.valve effective when deenergized to cause application of the brakes ofthetrain, a plurality of low-pass filters for different frequenciescomparable to the frequencies of said generator at predetermined trainspeeds, train control receiving apparatus operable in response towayside apparatus, said receiving apparatus being effective to 'select'one of said low-pass filters, an oscillator, a first detector relay andan associated repeater relay, circuit means for energizing said firstdetector relay by the output from said generator, and from the output ofsaid oscillator, means responsive to the actuation of said repeaterrelay to a particular position for rendering said oscillator ineffectiveto apply an output to said circuit means, whereby said first detectorrelay is steadily energized when there is an output from said generatorand is pulsed when there is no output from said generator, a seconddetector relay, circuit means for energizing said second detector relayby the output of said generator through a contact of said first detectorrelay and the low-pass filter selected by said receiving apparatus, andcircuit means for maintaining said electropneumatic valve energized inresponse to the intermittent operation of said repeater relay and inresponse to the energization of said second detector relay.

12. A train speed control system having train carried apparatuscomprising in combination, an axle driven frequency generator, anelectro-pneumatic valve effective when deenergized to cause applicationof the brakes of the train, a condenser, a slow drop away relayeffective when picked up to energize said electro-pneumatic valve, saidslow drop away relay being subject to energization only by the dischargeof said condenser, a high-pass filter, an oscillator effective togenerate a frequency above the pass band of said high-pass filter, acondenser chargedischarge control relay having contacts selectingrespectively the charge of said condenser and the discharge of saidcondenser across the winding of said slow drop away relay,electro-responsive means effective to energize said charge-dischargecontrol relay intermittently by energy from said oscillator feedingthrough said high-pass filter, said electro-responsive means beingeffective to steadily energize said charge-discharge control relay byenergy from said generator feeding through said filter when thefrequency of the generator is above the frequency of the high-passfilter, whereby the steady energization of said charge-discharge controlrelay by reason of the frequency of the frequency generator being withinthe pass band of said high-pass filter causes the dropping away of saidslow drop away relay and the actuation of said electro-pneumatic valveto apply the brakes of the train.

13. A train speed control system having train carried apparatuscomprising in combination, an axle driven frequency generator, anelectro-pneumatic brake control valve effective when deenergized tocause the application of the brakes of the train, a control relay forsaid brake control valve efiective when energized to maintain saidelectro-pneumatic brake control valve energized and effective whendeenergized to cause the deenergization of said electro-pneumatic brakecontrol valve, electro-responsive receiving apparatus distinctivelyactuated in accordance with the character of energization of the trackrails, a high-pass filter, and circuit means including said high-passfilter for maintaining said control relay picked up to thereby energizesaid electro-pneumatic valve except when the frequency of said frequencygenerator is at a frequency within the pass band of said high-pass 14.In a train control system, train carried apparatus comprising, and axledriven frequency generator, an electro-pneumatic valve effective whendeenergized to cause application of the brakes of the train, a filtereffective to block frequencies outside of a particular frequency range,a detector relay having a slow acting repeater relay, an oscillatorhaving its output applied to a Winding of said generator, meanseffective upon the actuation of said repeater relay for rendering saidoscillator ineffective to provide an output to said winding of saidgenerator, circuit means including a winding of said generator and saidfilter for energizing said detector relay and thereby actuate said slowacting relay to render the output of said oscillatorineffective, acontrol relay, circuit means for maintaining said control relay pickedup only when said repeater relay is actuated at frequent intervals, andcircuit means dependent upon said control relay being picked up forenergizing said electropneumatic valve.

15. In a train control system, train carried apparatus comprising, anaxle driven frequency generator, an electro-pneumatic valve effectivewhen deenergized to cause application of the brakes of the train, aplurality of filters, each filter being effective to block a differentrange of frequencies, track circuit code receiving means effective toselect one of said filters dependent upon the particular track circuitcode received, a detector relay having a slow acting repeater relay,circuit means including the filter selected by said track code receivingmeans for energizing said detector relay from the output of saidgenerator, an oscillator having its output applied to said circuitmeans, means effective upon the actuation of said repeater relay forrendering said oscillator ineffective to provide an output to saidcircuit means to thereby cause actuation of said repeater relayintermittently except when said detector relay is steadily energized bythe output of said frequency generator, a control relay, circuit meansfor maintaining said control relay picked up provided that said repeaterrelay is intermittently actuated, and circuit means for energizing saidelectro-pneumatic valve provided that said control relay is picked up.

16. In a train control system, train carried apparatus comprising, anaxle driven frequency generator, an electro-pneumatic valve effectivewhen deenergized to cause application of the brakes of the train, aplurality of highpass filters respectively effective to pass currentabove different frequencies, track circuit code receiving meanseffective to select one of said filters dependent upon the particulartrack circuit code received, a filter having a lower frequency pass bandbeing selected for a lower rate code, a detector relay having a slowacting repeater relay, circuit means including the filter selected bysaid receiving means for energizing said detector relay from the outputof said generator, an oscillator generating a frequency high enough tofall within the pass band of the selected high-pass filter, saidoscillator having its output applied to said circuit means to check theintegrity of the circuit means for the energization of said detectorrelay from said generator, means responsive to the actuation of saidrepeater relay for rendering said oscillator ineffective to provide anoutput to said circuit means, whereby said repeater relay isintermittently actuated by reason of energy originating at saidoscillator, a control relay, circuit means for maintaining said controlrelay picked up provided that said repeater relay is intermittentlyactuated, and circuit means for energizing said electro-pneumatic valveprovided that said control relay is picked up.

17. In a train control system, train carried apparatus comprising, anaxle driven frequency generator, an electro-pneumatic valve effectivewhen deenergized to cause application of the brakes of the train, aplurality of filters, each filter being effective to block a differentrange of frequencies, train control receiving apparatus effective to beconditioned inductively by wayside apparatus, said apparatus beingeffective to select one of said filters, a detector relay having a slowacting repeater relay, circuit means including the filter selected bysaid receiving apparatus for energizing said detector relay as a resultof the output of said generator when the output of the generator is highenough to fall within the pass band of the selected filter, anoscillator generating a frequency within the pass band of said selectedfilter having its output applied to said circuit means for checking theintegrity of the circuit means for the energization of said detectorrelay as a result of the output of said generator, means responsive tothe actuation of said repeater relay for rendering said oscillatorineffective to provide an output to said circuit means, whereby saidrepeater relay is intermittently actuated as a result of the output ofsaid oscillator and is steadly picked up as a result of a frequencyoutput of said generator high enough to fall into the pass band of theselected filters, a control relay, circuit means for maintaining saidcontrol relay picked up provided that said repeater relay isintermittently actuated and circuit means for energizing saidelectropneumatic valve provided that said control relay is picked up.

18. In a train control system, train carried apparatus comprising, anaxle driven frequency generator, an electropneumatic valve effectivewhen deenergized to cause application of the brakes of the train, a highpass filter effective to pass frequencies above a particular frequencycomparable to a speed limit frequency of said axle driven generator, adetector relay having a slow acting repeater relay, circuit meansincluding said filter for energizing said detector relay in response tothe output of said generator when the frequency of the generator isabove the frequency of said filter, an oscillator generating a frequencywithin the pass band of said filter, said oscillator having its outputapplied to said circuit means to thereby govern said detector relay,means responsive to the actuation of said repeater relay for renderingsaid oscillator ineffective to provide an output to said circuit means,whereby said repeater relay is subject to intermittent actuation inresponse to the output of said oscillator and is subject to steadyenergization when the frequency of said axle driven generator is highenough to fall into the pass band of said filter, and circuit means formaintaining said electro-pneumatic valve energized provided that saiddetector relay is actuated at frequent intervals.

19. A train control system having train carried apparatus comprising incombination, an axle driven frequency generator, an electropneumaticvalve effective when deenergized to cause application of the brakes of atrain, a plurality of filters for blocking different ranges offrequencies respectively, track circuit code receiving means eifectiveto select one of said filters in accordance with the particular trackcircuit code received, a detector relay having a slow acting repeaterrelay associated therewith, an oscillator, circuit means for energizingsaid detector relay by the output generated by said frequency generatorwhen it is being driven, and said circuit means being effective whensaid generator is not being driven to intermittently energize saiddetector relay in accordance with the output of said oscillator, theoutput of said oscillator being intermittently opened by a contact ofsaid slow acting repeater relay, control means for governing theenergization of said electropneumatic valve, said control means beingeffective to maintain said electropneumatic valve steadily energizedwhen said generator is not being driven in accordance with theintermittent operation of said detector relay, and said control meansbeing effective when said generator is being driven to deenergize saidelectropneumatic valve when the frequency of the output of saidgenerator exceeds a limit determined by a selected one of said filters.

References Cited in the file of this patent UNITED STATES PATENTS1,800,972 Williams et a1. Apr. 14, 1931 2,222,801 Logan Nov. 26, 19402,559,397 Bushnell July 3, 1951 2,606,281 Thomas Aug. 5. 1952

